1. Field of the Invention
The present invention relates generally to nonvolatile data storage systems, and more particularly to such systems having storage mechanisms including read/write heads that are indexed and precisely positioned via encoded servo sectors.
2. Description of the Background Art
Designers, manufacturers, and users of computing systems require reliable and efficient digital information storage and retrieval equipment. Conventional magnetic disk drive storage systems have been commonly used and are well known in the art. These storage systems typically use a flying magnetic read/write head to record and retrieve data from a layer of magnetic recording material on the surface of a rotating recording disk. The capacity of such a storage system is a function of the number of closely spaced concentric tracks on the recording disk that may be reliably accessed by the read/write head. Some of the recording disk surface area is used for purposes other than data storage, however.
Means for assuring the proper selection of a particular track by the read/write head are required for reliable data storage and retrieval. The read/write head should also be kept centered over a particular track as the recording disk rotates, to prevent accidental over-writing of data stored in neighboring tracks. Some systems use nonmagnetic guard rings between discrete tracks on the recording disk to help keep the head from skipping off-track. Gain control references should be placed at different locations on the recording disk to calibrate the electronic amplifiers used to reliably read back data signals. Time delays are also needed to allow the magnetic read/write head to demagnetize after recording data, to prevent unintentional over-writing of subsequently accessed locations. The designs created to accomplish these goals take up some of the available recording disk surface area, and thus reduce overall system capacity.
Various types of indexing marks and alignment indicia are also recorded on the recording disk surface for precise position reference and tracking adjustment of the read/write head. These marks and indicia are often recorded in servo sectors, which are angularly-spaced reserved portions of the recording disk surface that extend out approximately radially from the recording disk centers. Track addresses are sometimes recorded in servo sectors. Angular synchronization signals that determine the circumferential location of the magnetic head may also be recorded in servo sectors. Normal and quadrature servo blocks are often recorded in servo sectors for generation of position error signals that are used to keep the read/write head aligned. Servo sectors use recording disk surface area that could otherwise be used for data storage, however, so servo sector information should be stored as efficiently as possible.
Newer magneto-optical technology offers many improvements over conventional magnetic technology, particularly in terms of increased capacity. Magneto-optical storage systems also record data onto a recording material coated onto the surfaces of one or more rotating recording disks, but via different means than conventional drives. The recording material undergoes a sharp increase in magnetic susceptibility when heated beyond its Curie point, the temperature at which the magnetic properties of the recording material change from ferromagnetic to paramagnetic. A localized magnetic domain is created by heating a region of the recording material and then applying a magnetic field of a desired orientation to the heated region. When the recording material cools, the localized magnetic domain retains its magnetic orientation and again becomes far less susceptible to applied magnetic fields.
An optical fiber may guide an intense beam of focused laser light to heat a localized magnetic domain to be recorded or overwritten. The data stored in a particular localized magnetic domain may also be read back nondestructively by such a combined laser and optical fiber system. A low-powered, linearly polarized laser beam focused on a particular localized magnetic domain will be reflected with a Kerr rotation of the angle of polarization determined by the magnetic orientation of the localized magnetic domain. The pattern of polarization rotations read back as the low-powered laser beam moves across the recording surface thus represents the pattern of magnetic orientations previously written onto the recording surface. The overall reflectivity of a localized magnetic domain may also be determined via measurement of the relative amplitude of the reflected laser beam.
Magneto-optical storage systems should quickly and reliably locate and align to any particular storage location on the recording disk, as with existing storage systems. A scheme for accomplishing these goals that takes advantage of the unique properties of a magneto-optical storage system is needed. An efficient system for encoding servo sector information is therefore important for maximizing the amount of remaining disk surface area available for data storage and retrieval.
In accordance with the present invention, a system and method are disclosed for efficiently encoding servo sector information in a data storage system using rotating magneto-optical recording disks. In the preferred embodiment, servo sectors are angularly-spaced portions of the recording disk surface that extend out radially from the disk centers and include position reference and tracking adjustment information for read/write heads. The fraction of the recording disk surface occupied by servo sectors should be minimized to maximize storage capacity.
Servo sector patterns are quickly stamped into recording surfaces during manufacture and can simultaneously improve many aspects of prior storage systems. Automatic gain control reference blocks of conventional magnetic storage systems are rendered unnecessary because the reflectivity of the servo sectors is uniform across the recording disk surface. Write-to-read recovery time, usually needed to allow a high-intensity recording laser beam to reduce power to the level used for reading data, is also unnecessary when servo sector patterns are indelible. Servo timing marks are preferably embossed into each of the servo sectors to help synchronize data storage and retrieval with the rotation of the recording disks. The servo timing marks may also serve as part of a system to detect defects on the recording disk surfaces. Encoded track address data and sector address data, and position error signal blocks are also preferably permanently affixed into each of the servo sectors to help the read/write heads locate and follow a particular track.
In the preferred embodiment, servo sector information is read via reflectivity measurement, not the magnetically-induced Kerr rotation measurement used for data storage and retrieval in magneto-optical systems. Since the servo timing marks therefore do not need to be discriminated from data, error correction efficiency is roughly doubled. Surface reflectivity between distinct servo timing marks preferably certifies the recording surface as reliable for writing, eliminating the need for read-back verification delays.
The radial seeking speed of the read/write head is increased in the preferred embodiment by use of only a few lower-order track address bits on every sector. The full track address is rarely needed because the target track is typically selected from only a small number of frequently-scanned neighboring tracks having almost identical addresses. Higher-order track address bits may be distributed across consecutive servo sectors because they are needed only as a less frequent confirmation that the lower-order track address bits are properly wrapped, that is, successfully ramped through neighboring low-to-high and high-to-low address transitions.
A repeating sequence of bits distributed around the circumference of the recording disk in the preferred embodiment verifies the servo sector number kept in a counter in a disk controller. The sequence of distributed bits also identifies the phase of the distributed higher-order track address bits for proper significance assignment. A timing mark to enable precise rotation synchronization by the disk controller preferably comprises either a higher-order track address bit or its complement, to guarantee that a positive mark will always be available for detection and use.
The present invention thus enables the storage system to quickly and reliably locate, certify, and follow any particular track and sector while maximizing the amount of disk surface area available for data storage and retrieval.